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Plasticsportal - About Plastics  

The plastixportal offers an about plastics page to have Plastic Terms explained - to cover plastic raw materials, machinery, and processes: Companies may feel free to add information here please send info@plastixportal.co.za,
see also Uses of Plastics

In the extrusion of plastics, raw thermoplastic material in the form of small beads (often called resin in the industry) is gravity fed from a top mounted hopper into the barrel of the extruder. Additives such as colorants and UV inhibitors (in either liquid or pellet form) are often used and can be mixed into the resin prior to arriving at the hopper.

The material enters through the feed throat (an opening near the rear of the barrel) and comes into contact with the screw. The rotating screw (normally turning at up to 120 rpm) forces the plastic beads forward into the barrel which is heated to the desired melt temperature of the molten plastic (which can range from 200 °C/400 °F to 275 °C/530 °F depending on the polymer). In most processes, a heating profile is set for the barrel in which three or more independent PID controlled heater zones gradually increase the temperature of the barrel from the rear (where the plastic enters) to the front. This allows the plastic beads to melt gradually as they are pushed through the barrel and lowers the risk of overheating which may cause degradation in the polymer.

Extra heat is contributed by the intense pressure and friction taking place inside the barrel. In fact, if an extrusion line is running a certain material fast enough, the heaters can be shut off and the melt temperature maintained by pressure and friction alone inside the barrel. In most extruders, cooling fans are present to keep the temperature below a set value if too much heat is generated. If forced air cooling proves insufficient then cast-in heater jackets are employed, and they generally use a closed loop of distilled water in heat exchange with tower or city water.

Plastic extruder cut in half to show the componentsAt the front of the barrel, the molten plastic leaves the screw and travels through a screen pack to remove any contaminants in the melt. The screens are reinforced by a breaker plate (a thick metal puck with many holes drilled through it) since the pressure at this point can exceed 5000 psi (34 MPa). The screen pack/breaker plate assembly also serves to create back pressure in the barrel. Back pressure is required for uniform melting and proper mixing of the polymer, and how much pressure is generated can be 'tweaked' by varying screen pack composition (the number of screens, their wire weave size, and other parameters). This breaker plate and screen pack combination also does the function of converting "rotational memory" of the molten plastic into "longitudinal memory".

After passing through the breaker plate molten plastic enters the die. The die is what gives the final product its profile and must be designed so that the molten plastic evenly flows from a cylindrical profile, to the product's profile shape. Uneven flow at this stage would produce a product with unwanted stresses at certain points in the profile. These stresses can cause warping upon cooling. Almost any shape imaginable can be created so long as it is a continuous profile.

The product must now be cooled and this is usually achieved by pulling the extrudate through a water bath. Plastics are very good thermal insulators and are therefore difficult to cool quickly. Compared with steel, plastic conducts its heat away 2000 times more slowly. In a tube or pipe extrusion line, a sealed water bath is acted upon by a carefully controlled vacuum to keep the newly formed and still molten tube or pipe from collapsing. For products such as plastic sheeting, the cooling is achieved by pulling through a set of cooling rolls.

Sometimes on the same line a secondary process may occur before the product has finished its run. In the manufacture of adhesive tape, a second extruder melts adhesive and applies this to the plastic sheet while it’s still hot. Once the product has cooled, it can be spooled, or cut into lengths for later use.

Screw design
There are five possible zones in a thermoplastic screw. Since terminology is not standardized in the industry, different names may refer to these zones. Different types of polymer will have differing screw designs, some not incorporating all of the possible zones.

A simple plastic extrusion screw
Feed zone. Also called solids conveying. This zone feeds the resin into the extruder.
Melt zone. Also called the transition zone. The resin is melted in this section.
Pressurizing zone. Also called metering or melt conveying. This zone gives the plastic uniform pressure and flow characteristics.
Decompression zone. In this zone, the melt is unpressurized, allowing trapped gases (hydrochloride) to escape and be vented out.
Mixing zone. There are two types of mixing zone. They either distribute small particles evenly, or break large particles into small ones which can then be mixed.
Often screw length is referenced to its diameter in terms of an L:D ratio. For instance, a 6-inch (150 mm) diameter screw at 24:1 will be 144 inches (12 ft) long, and 192 inches (16 ft) at 32:1. In years past screw ratios of 24:1 were fairly common, but modern machines use 32:1, or higher ratios, which allow better mixing at higher throughput.

Each zone will be equipped with a one or more thermocouples or RTDs for temperature control.

Geometrical Possibilities
There are many geometrical possibilities when using extrusion as a method to process plastics. Simple round pipes and square shapes are common. More complex shapes, such as tracks and profiles, are also commonly produced by extrusion molding. Solid extrusions can be produced with a large thickness. Wall thicknesses of hollow extrusions typically vary from 0.005 in. to around 0.3 in. The Length is typically anywhere between 10 ft and 100 ft., although theoretically if you had enough room and material you could produce an extrusion with an infinitely long length.

Sheet/film extrusion
For products such as plastic sheet or film, the cooling is achieved by pulling through a set of cooling rolls (calender or "chill" rolls), usually 3 or 4 in number. Running too fast creates an undesirable condition called "nerve"- basically, inadequate contact time is allowed to dissipate the heat present in the extruded plastic. In sheet extrusion, these rolls not only deliver the necessary cooling but also determine sheet thickness and surface texture (in case of structured rolls; i.e. smooth, levant, haircell, etc.).

Often co-extrusion is used to apply one or more layers to obtain specific properties such as UV-absorption, soft touch or "grip", matte surface, or energy reflection.

A common post-extrusion process for plastic sheet stock is thermoforming, where the sheet is heated until soft (plastic), and formed via a mold into a new shape. When vacuum is used, this is often described as vacuum forming. Orientation (i.e. ability/ available density of the sheet to be drawn to the mold which can vary in depths from 1 to 36 inches typically) is highly important and greatly affects forming cycle times.

Thermoforming can go from line bended pieces (e.g. displays) to complex shapes (computer housings), which often look like being injection moulded, thanks to the various possibilities in thermoforming, such as inserts, undercuts, divided moulds.

Plastic extrusion onto paper is the basis of the liquid packaging industry (juice cartons, wine boxes...); usually an aluminum layer is present as well. In food packaging plastic film is sometimes metallised, see metallised film.

Blown film extrusion
The manufacture of plastic film for products such as shopping bags is achieved using a blown film line.

This process is the same as a regular extrusion process up until the die. The die is an upright cylinder with a circular opening similar to a pipe die. The diameter can be a few centimetres to more than three metres across. The molten plastic is pulled upwards from the die by a pair of nip rolls high above the die (4 metres to 20 metres or more depending on the amount of cooling required). Changing the speed of these nip rollers will change the gauge (wall thickness) of the film. Around the die sits an air-ring. The air-ring cools the film as it travels upwards. In the centre of the die is an air outlet from which compressed air can be forced into the centre of the extruded circular profile, creating a bubble.This expands the extruded circular cross section by some ratio (a multiple of the die diameter). This ratio, called the “blow-up ratio” can be just a few percent to more than 200 percent of the original diameter. The nip rolls flatten the bubble into a double layer of film whose width (called the “layflat”) is equal to ½ the circumference of the bubble. This film can then be spooled or printed on, cut into shapes, and heat sealed into bags or other items.

An advantage of blown film extrusion over traditional film extrusion is that in the latter there are edges where there can be quality (thickness,... ) variations.

Overjacketing extrusion
In a wire coating process, bare wire (or bundles of jacketed wires, filaments, etc) is pulled through the center of a die similar to a tubing die. Many different materials are used for this purpose depending on the application. Essentially, an insulated wire is a thin walled tube which has been formed around a bare wire.

There are two different types of extrusion tooling used for coating over a wire. They are referred to as either "pressure" or "jacketing" tooling. The selection criteria for choosing which type of tooling to use is based on whether the particular application requires intimate contact or adhesion of the polymer to the wire or not. If intimate contact or adhesion is required, pressure tooling is used. If it is not desired, jacketing tooling is chosen.

The main difference in jacketing and pressure tooling is the position of the pin with respect to the die. For jacketing tooling, the pin will extend all the way flush with the die. When the bare wire is fed through the pin, it does not come in direct contact with the molten polymer until it leaves the die. For pressure tooling, the end of the pin is retracted inside the crosshead, where it comes in contact with the polymer at a much higher pressure.

Tubing extrusion
Plastic tubing, such as drinking straws and medical tubing, is manufactured by extruding molten polymer through a die of the desired profile shape (square, round, triangular). Hollow sections are usually extruded by placing a pin or mandrel inside of the die, and in most cases positive pressure is applied to the internal cavities through the pin.

Sometimes tubing with multiple lumens (holes) must be made for specialty applications. For these applications, the tooling is made by placing more than one pin in the center of the die, to produce the number of lumens necessary. In most cases, these pins are supplied with air pressure from different sources. In this way, the individual lumen sizes can be adjusted by adjusting the pressure to the individual pins.

Coextrusion
Coextrusion refers to the extrusion of multiple layers of material simultaneously. This type of extrusion utilizes two or more extruders to melt and deliver a steady volumetric throughput of different viscous plastics to a single extrusion head (die) which will extrude the materials in the desired form. This technology is used on any of the processes described above (Blown Film, Overjacketing, Tubing, Sheet). The layer thicknesses are controlled by the relative speeds and sizes of the individual extruders delivering the materials.

There are a variety of reasons a manufacturer may choose coextrusion over single layer extrusion. One example is in the vinyl fencing industry, where coextrusion is used to tailor the layers based on whether they are exposed to the weather or not. Usually a thin layer of compound that contains expensive weather resistant additives are extruded on the outside while the inside has an additive package that is more suited for impact resistance and structural performance.

Extrusion coating
Extrusion coating is using a blown or cast film process to coat an additional layer onto an existing rollstock of paper, foil or film. For example, this process can be used to improve the characteristics of paper by coating it with polyethylene to make it more resistant to water. The extruded layer can also be used as an adhesive to bring two other materials together. A famous product that uses this technology is tetrapak.

Typical workpiece materials
Typical materials that are used in extrusion molding include but are not limited to: Acetal, acrylic, nylon, polystyrene (which are the best materials) and acrylonitrile butadiene styrene (ABS) and polycarbonate(which aren’t as moldable)

Injection moulding machine
A plastic injection moulding machine for making plastic parts. Manufacturing products by injection moulding process. Consist of two main parts, an injection unit and a clamping unit. Injection moulding machines can fasten the moulds in either a horizontal or vertical position. The majority is horizontally oriented but vertical injection moulding machines are used in some niche applications such as insert moulding, allowing the plastic injection moulding machine to take advantage of gravity. There are many ways to fasten the tools to the platens, the most common being manual clamps (both halves are bolted to the platens); however hydraulic clamps (chocks are used to hold the tool in place) and magnetic clamps are also used. The magnetic and hydraulic clamps are used where fast tool changes are required.

Types of injection moulding machines
Plastic injection moulding machines are classified primarily by the type of driving systems they use: hydraulic, electric, or hybrid. Hydraulic presses have historically been the only option available to moulders until Nissei Plastic Industrial Co., LTD introduced the first all-electric injection moulding machine in 1983. The electric press, also known as Electric Machine Technology (EMT), reduces operation costs by cutting energy consumption and also addresses some of the environmental concerns surrounding the hydraulic press. Electric presses have been shown to be quieter, faster, and have a higher accuracy, however the machines are more expensive. Hybrid injection moulding machines take advantage of the best features of both hydraulic and electric systems. Hydraulic machines are the predominant type in most of the world. Hydraulic Injection Moulding Machines, Electric Injection Moulding Machines, Toggle Injection Moulding Machines.

Raw Materials
Polyurethanes : Polyurethanes are formed when two chemicals - a diisocyanate and a polyol, are mixed together in the presence of suitable catalysts and activators.

Rigid Polyurethane Foams are characterised by their outstanding insulation properties that can be utilised in many different ways. Systems may be poured, injected or sprayed according to the application requirements. The construction of modern domestic refrigerators and freezers would not be possible without the use of polyurethane foam. While its high insulation efficiency ensures minimal wall thickness, the polyurethane also acts as the structural adhesive to bond the inner and outer skins of the refrigerator during the foaming process. In the construction industry, similar technology is used to produce laminated rigid foam panels for the manufacture of industrial roofsand walls that, through their excellent insulation properties, help to maintain a constant and comfortable working environment whatever the season. Integral skin mouldings are also produced from rigid foams for structural and decorative purposes in the furniture industry. The excellent definition properties of the foams allow high quality simulated wood articles to be easily and cost-effectively fabricated.

Flexible Polyurethane Foams may be formulated across a broad range of hardnesses and densities to accomodate thousands of end applications. They can be produced in a variety of shapes by either cutting from slabstock or by moulding, with their main uses being in upholstered furniture, mattresses and automotive seating. For many years ozone depleting chemicals such as the CFC's and HCFC's were widely used as blowing agents, today more environmental acceptable blowing agents are used.

Solid Elastomers many types of solid elastomers are available, including thermoplastics for use on conventional thermoplastic processing equipment, castable systems for hand and machine dispensing and solvent-free srayable systems for high build coatings.The major applications are based upon the outstanding abrasion resistance of polyurethane elastomers, as well as their resistance to solvents and chemicals. Other properties such as low temperature flexibility or resistance to microbiological attack can also be designed into the polymer where required. The excellent adhesion characteristics of polyurethanes can bind diverse materials. These include foundry sand for the metal casting industry, timber particles and other by-products for wood composite panels, rubber"crumb" for playing surfaces for athletics and other sports, safety flooring and foam chips for low-cost cushioning.

The above information - courtesy of Industrial Urethanes

Information about Plastic Raw Materials :
PET
Polyethylene terephthalate (aka PET, PETE or the obsolete PETP or PET-P) is a thermoplastic polymer resin of the polyester family and is used in synthetic fibers; beverage, food and other liquid containers; thermoforming applications; and engineering resins often in combination with glass fiber. It is one of the most important raw materials used in man-made fibers.

Depending on its processing and thermal history, it may exist both as an amorphous (transparent) and as a semi-crystalline (opaque and white) material. Its monomer can be synthesized by the esterification reaction between terephthalic acid and ethylene glycol with water as a byproduct, or the transesterification reaction between ethylene glycol and dimethyl terephthalate with methanol as a byproduct. Polymerization is through a polycondensation reaction of the monomers (done immediately after esterification/transesterification) with ethylene glycol as the byproduct (the ethylene glycol is recycled in production).

The majority of the world's PET production is for synthetic fibers (in excess of 60%) with bottle production accounting for around 30% of global demand. In discussing textile applications, PET is generally referred to as simply "polyester" while "PET" is used most often to refer to packaging applications.

PET Applications:
PET can be semi-rigid to rigid, depending on its thickness, and is very lightweight. It makes a good gas and fair moisture barrier, as well as a good barrier to alcohol (requires additional "Barrier" treatment) and solvents. It is strong and impact-resistant. It is naturally colorless and transparent.

When produced as a thin film (often known by the tradename Mylar), PET is often coated with aluminium to reduce its permeability, and to make it reflective and opaque. PET bottles are excellent barrier materials and are widely used for soft drinks, (see carbonation). PET or Dacron is also used as a thermal insulation layer on the outside of the International Space Station as seen in an episode of Modern Marvels "Sub Zero". For certain specialty bottles, PET sandwiches an additional polyvinyl alcohol to further reduce its oxygen permeability.

When filled with glass particles or fibers, it becomes significantly stiffer and more durable. This glass-filled plastic, in a semi-crystalline formulation, is sold under the tradename Rynite, Arnite, Hostadur& Crastin.

Sails are usually made of Dacron, a brand of PET fiber; colorful lightweight spinnakers are usually made of nylon.While all thermoplastics are technically recyclable, PET bottle recycling is more practical than many other plastic applications. The primary reason is that plastic carbonated soft drink bottles and water bottles are almost exclusively PET which makes them more easily identifiable in a recycle stream. PET has a resin identification code of 1. PET, as with many plastics, is also an excellent candidate for thermal recycling (incineration) as it is composed of carbon, hydrogen and oxygen with only trace amounts of catalyst elements (no sulfur) and has the energy content of soft coal. PET was patented in 1941 by the Calico Printers' Association of Manchester. The PET bottle was patented in 1973. Copolymers
In addition to pure (homopolymer) PET, PET modified by copolymerization is also available. In some cases, the modified properties of copolymer are more desirable for a particular application. For example, cyclohexane dimethanol (CHDM) can be added to the polymer backbone in place of ethylene glycol. Since this building block is much larger (6 additional carbon atoms) than the ethylene glycol unit it replaces, it does not fit in with the neighboring chains the way an ethylene glycol unit would. This interferes with crystallization and lowers the polymer's melting temperature. Such PET is generally known as PETG (EastmanChemical and SKchemicals are the only two manufacturers).
Replacing terephthalic acid (right) with isophthalic acid (center) creates a kink in the PET chain, interfering with crystallization and lowering the polymer's melting point.Another common modifier is isophthalic acid, replacing some of the 1,4- (para-) linked terephthalate units. The 1,2- (ortho-) or 1,3- (meta-) linkage produces an angle in the chain, which also disturbs crystallinity. Such copolymers are advantageous for certain molding applications, such as thermoforming, which is used to make tray or blister packages from PET sheet (sometimes called APET, for "amorphous PET"). On the other hand, crystallization is important in other applications where mechanical and dimensional stability are important, such as seat belts. For PET bottles, the use of small amounts of CHDM or other comonomers can be useful: if only small amounts of comonomers are used, crystallization is slowed but not prevented entirely. As a result, bottles are obtainable via stretch blow molding ("SBM"), which are both clear and crystalline enough to be an adequate barrier to aromas and even gasses, such as the carbon dioxide in carbonated beverages.
LDPE
Low-density polyethylene (LDPE) is a thermoplastic made from oil. It was the first grade of polyethylene, produced in 1933 by Imperial Chemical Industries (ICI) using a high pressure process via free radical polymerisation [1]. Its manufacture employs the same method today.
Applications:
LDPE is widely used for manufacturing various containers, dispensing bottles, wash bottles, tubing, plastic bags for computer components, and various molded laboratory equipment. Its most common use is in plastic bags.
Other products made from it include:
Trays & general purpose containers
Food storage and laboratory containers
Corrosion-resistant work surfaces
Parts that need to be weldable and machinable
Parts that require flexibility, for which it serves very well
Very soft and pliable parts
Six-pack soda can rings
Extrusion coating on paperboard and aluminum laminated for beverage cartons.
Computer components, such as hard drives, screen cards and disk-drives.

Acrylonitrile butadiene styrene
Monomers in ABS polymerAcrylonitrile butadiene styrene, or ABS, (chemical formula (C8H8· C4H6·C3H3N)n) is a common thermoplastic used to make light, rigid, molded products such as piping, musical instruments (most notably recorders), golf club heads (used for its good shock absorbance), automotive body parts, wheel covers, enclosures, protective head gear, vballs [reusable paintballs], and toys including LEGO bricks[1]. In plumbing, ABS pipes are the black pipes (PVC pipes are white) and also in Plastic Pressure Pipe Systems. ABS plastic ground down to an average diameter of less than 1 micrometre is used as the colorant in some tattoo inks. Tattoo inks that use ABS are extremely vivid. This vividness is the most obvious indicator that the ink contains ABS, as tattoo inks rarely list their ingredients. It is a copolymer made by polymerizing styrene and acrylonitrile in the presence of polybutadiene. The proportions can vary from 15 to 35% acrylonitrile, 5 to 30% butadiene and 40 to 60% styrene. The result is a long chain of polybutadiene criss-crossed with shorter chains of poly(styrene-co-acrylonitrile). The nitrile groups from neighbouring chains, being polar, attract each other and bind the chains together, making ABS stronger than pure polystyrene. The styrene gives the plastic a shiny, impervious surface. The butadiene, a rubbery substance, provides resilience even at low temperatures. ABS can be used between -25 and 60 °C. Production of 1 kg of ABS requires the equivalent of about 2 kg of oil for raw materials and energy. It can also be recycled.

PVC
Polyvinyl chloride, (IUPAC Polychloroethene) commonly abbreviated PVC, is a widely used thermoplastic polymer. In terms of revenue generated, it is one of the most valuable products of the chemical industry. Around the world, over 50% of PVC manufactured is used in construction. As a building material, PVC is cheap, durable, and easy to assemble. In recent years, PVC has been replacing traditional building materials such as wood, concrete and clay in many areas. Polyvinyl chloride is used in a variety of applications. As a hard plastic, it is used as vinyl siding, magnetic stripe cards, window profiles, gramophone records (which is the source of the term vinyl records), pipe, plumbing and conduit fixtures. The material is often used in Plastic Pressure Pipe Systems for pipelines in the water and sewer industries because of its inexpensive nature and flexibility. PVC pipe plumbing is typically white, as opposed to ABS, which is commonly available in grey and black, as well as white. It can be made softer and more flexible by the addition of plasticizers, the most widely-used being phthalates. In this form, it is used in clothing and upholstery, and to make flexible hoses and tubing, flooring, to roofing membranes, and electrical cable insulation.


All information above reproduced from Wikipedia®

Injection Moulding - All About:
Injection molding (British variant spelling: moulding) is a manufacturing technique for making parts from both thermoplastic and thermosetting plastic materials in production. Molten plastic is injected at high pressure into a mold (British variant spelling: mould), which is the inverse of the product's shape. After a product is designed by an Industrial Designer or an Engineer, molds are made by a moldmaker (or toolmaker) from metal, usually either steel or aluminium, and precision-machined to form the features of the desired part. Injection molding is widely used for manufacturing a variety of parts, from the smallest component to entire body panels of cars. Injection molding is the most common method of production, with some commonly made items including bottle caps and outdoor furniture.
Materials Used:
The most commonly used thermoplastic materials are polystyrene (low cost, lacking the strength and longevity of other materials), ABS or acrylonitrile butadiene styrene (a co-polymer or mixture of compounds used for everything from Lego parts to electronics housings), nylon (chemically resistant, heat resistant, tough and flexible - used for combs), polypropylene (tough and flexible - used for containers), polyethylene, and polyvinyl chloride or PVC (more common in extrusions as used for pipes, window frames, or as the insulation on wiring where it is rendered flexible by the inclusion of a high proportion of plasticiser). Injection molding can also be used to manufacture parts from aluminium or brass. The melting points of these metals are much higher than those of plastics; this makes for substantially shorter mold lifetimes despite the use of specialized steels. Nonetheless, the costs compare quite favorably to sand casting, particularly for smaller parts.

Equipment:
Injection molding machines, also known as presses, hold the molds in which the components are shaped. Presses are rated by tonnage, which expresses the amount of clamping force that the machine can generate. This pressure keeps the mold closed during the injection process. Tonnage can vary from less than 5 tons to 6000 tons, with the higher figures used in comparatively few manufacturing operations.

Plastic Injection Mould (Mold)
Mold (Tool and/or Mold) is the common term used to describe the production tooling used to produce plastic parts in injection molding. Traditionally, molds have been expensive to manufacture. They were usually only used in mass production where thousands of parts were being produced. Molds are typically constructed from hardened steel, pre-hardened steel, aluminium, and/or beryllium-copper alloy. The choice of material to build a mold is primarily one of economics. Steel molds generally cost more to construct, but their longer lifespan will offset the higher initial cost over a higher number of parts made before wearing out. Pre-hardened steel molds are less wear resistant and are used for lower volume requirements or larger components. The steel hardness is typically 38-45 on the Rockwell-C scale. Hardened steel molds are heat treated after machining. These are by far the superior in terms of wear resistance and lifespan. Typical hardness ranges between 50 and 60 Rockwell-C (HRC). Aluminium molds can cost substantially less, and when designed and machined with modern computerized equipment, can be economical for molding tens or even hundreds of thousands of parts. Beryllium copper is used in areas of the mold which require fast heat removal or areas that see the most shear heat generated. High performance alloys such as MoldMax® and Ampcoloy® have also been developed especially for optimum heat transfer. Such alloys are considered in mold construction when conventional heat removal methods are unsuitable or when cycle time is a critical consideration. Considerable thought is put into the design of molded parts and their molds, to ensure that the parts will not be trapped in the mold, that the molds can be completely filled before the molten resin solidifies, to compensate for material shrinkage, and to minimize imperfections in the parts.

Mould Design
Molds separate into at least two halves (called the core and the cavity) to permit the part to be extracted. In general the shape of a part must not cause it to be locked into the mold. For example, sides of objects typically cannot be parallel with the direction of draw (the direction in which the core and cavity separate from each other). They are angled slightly (draft), and examination of most plastic household objects will reveal this. Parts that are "bucket-like" tend to shrink onto the core while cooling, and after the cavity is pulled away. Pins are the most popular method of removal from the core, but air ejection, and stripper plates can also be used depending on the application. Most ejection plates are found on the moving half of the tool, but they can be placed on the fixed half. More complex parts are formed using more complex molds, which may have movable sections called slides which are inserted into the mold to form features that cannot be formed using only a core and a cavity. Slides are then withdrawn to allow the part to be released. Some molds allow previously molded parts to be reinserted to allow a new plastic layer to form around the first part. This is often referred to as overmolding. This system can allow for production of one piece tires and wheels. 2-shot or multi shot molds are designed to "overmold" within a single molding cycle and must be processed on specialized injection molding machines with two or more injection units. This can be achieved by having pairs of identical cores and pairs of different cavities within the mold. After injection of the first material, the component is rotated on the core from the one cavity to another. The second cavity differs from the first in that the detail for the second material is included. The second material is then injected into the additional cavity detail before the completed part is ejected from the mold. Common applications include "soft-grip" toothbrushes and freelander grab handles. The core and cavity, along with injection and cooling hoses form the mold tool. While large tools are very heavy (up to 60t), they can be hoisted into molding machines for production and removed when molding is complete or the tool needs repairing. A mold can produce several copies of the same parts in a single "shot". The number of "impressions" in the mold of that part is referred to as cavitation. A tool with one impression will often be called a single cavity (impression) tool. A mold with 2 or more cavities of the same parts will likely be referred to as multiple cavity tooling. Some extremely high production volume molds (like those for bottle caps) can have over 128 cavities. In some cases multiple cavity tooling will mold a series of different parts in the same tool. Some toolmakers call these molds family molds as all the parts are not the same but often part of a family of parts (to be used in the same product for example). Plastic Injection Moulding Process

Small injection molder showing hopper, nozzle and die area
[edit] Injection Molding Cycle
The basic injection cycle is as follows: Mold close - injection carriage forward - inject plastic - metering - carriage retract - mold open - eject part(s). The molds are closed shut, by hydraulics or electric, and the heated plastic is forced by the pressure of the injection screw to take the shape of the mold. Some machines are run by electric motors instead of hydraulics or a combination of both. The water-cooling channels then assist in cooling the mold and the heated plastic solidifies into the part. Improper cooling can result in distorted molding or one that is burnt. The cycle is completed when the mold opens and the part is ejected with the assistance of ejector pins within the mold.

The resin, or raw material for injection molding, is usually in pellet or granule form, and is melted by heat and shearing forces shortly before being injected into the mold.Resin pellets are poured into the feed hopper, a large open bottomed container, which feeds the granules down to the screw. The screw is rotated by a motor, feeding pellets up the screw's grooves. The depth of the screw flights decreases towards the end of the screw nearest the mold, compressing the heated plastic. As the screw rotates, the pellets are moved forward in the screw and they undergo extreme pressure and friction which generates most of the heat needed to melt the pellets. Heaters on either side of the screw assist in the heating and temperature control during the melting process. The channels through which the plastic flows toward the chamber will also solidify, forming an attached frame. This frame is composed of the sprue, which is the main channel from the reservoir of molten resin, parallel with the direction of draw, and runners, which are perpendicular to the direction of draw, and are used to convey molten resin to the gate(s), or point(s) of injection. The sprue and runner system can be cut or twisted off and recycled, sometimes being granulated next to the mold machine. Some molds are designed so that the part is automatically stripped through action of the mold. Blow Moulding
Blow molding or blow moulding (see spelling differences) is a manufacturing process by which hollow plastic parts are formed. In general, there are three main types of blow molding; Extrusion Blow Molding, Injection Blow Molding, and Stretch Blow Molding.

Injection blow moulding (molding)
The process of Injection Blow Molding (IBM) is used for the production of hollow glass and plastic objects in large quantities. In the IBM process, the polymer is injection molded onto a core pin; then the core pin is rotated to a blow molding station to be inflated and cooled. This is the least-used of the three blow molding processes, and is typically used to make small medical and single serve bottles. The process is divided into three steps: injection, blowing and ejection.

The injection blow molding machine is based on an extruder barrel and screw assembly which melts the polymer. The molten polymer is fed into a manifold where it is injected through nozzles into a hollow, heated preform mold. The preform mold forms the external shape and is clamped around a mandrel (the core rod) which forms the internal shape of the preform. The preform consists of a fully formed bottle/jar neck with a thick tube of polymer attached, which will form the body.

The preform mold opens and the core rod is rotated and clamped into the hollow, chilled blow mold. The core rod opens and allows compressed air into the preform, which inflates it to the finished article shape.

After a cooling period the blow mold opens and the core rod is rotated to the ejection position. The finished article is stripped off the core rod and leak-tested prior to packing. The preform and blow mold can have many cavities, typically three to sixteen depending on the article size and the required output. There are three sets of core rods, which allow concurrent preform injection, blow molding and ejection.

Another application of injection blow molding is in the production of soft elastic gelatin capsules for pharmaceutical applications. Two strips of gelatin are pressed together in a rotary die which cuts out the desired shape of capsule while the fill liquid is injected. Afterwards, they are cooled and dried to yield a firm, strong capsule.

Stretch blow moulding (molding)
In the Stretch Blow Molding (SBM) process, the plastic is first molded into a "preform" using the Injection Molded Process. These preforms are produced with the necks of the bottles, including threads (the "finish") on one end. These preforms are packaged, and fed later (after cooling) into an EBM blow molding machine. In the SBM process, the preforms are heated (typically using infrared heaters) above their glass transition temperature, then blown using high pressure air into bottles using metal blow molds. Usually the preform is stretched with a core rod as part of the process. The stretching of some polymers, such as PET (Polyethylene terephthalate) results in strain hardening of the resin, allowing the bottles to resist deforming under the pressures formed by carbonated beverages, which typically approach 60 psi.

The stretch blow molding process. The main applications are bottles, jars and other containers. The Injection blow molding process produces bottles of superior visual and dimensional quality compared to extrusion blow molding. The process is ideal for both narrow and wide-mouthed containers and produces them fully finished with no flash. A sign of injection blow molding is the seam where the two halves of the mold meet.

This picture shows what happens inside the blow mold. The preform is first stretched mechanically with a stretch rod. As the rod travels down low-pressure air of 5 to 25 bar (70 to 350 psi) is introduced blowing a 'bubble'. Once the stretch rod is fully extended, high-pressure air of up to 40 bar (580 psi) blows the expanded bubble into the shape of the blow mold.

Blow molding remained a relatively small part of the plastics manufacturing scene until the introduction of Low Density Polyethylene (LDPE) in the 1940's. The production of LDPE squeeze bottles by Monsanto caused a rapid expansion of the industry, with containers produced to replace glass bottles for shampoos and liquid soaps.

The mass production of high density polyethylene (HDPE) and polypropylene (PP) in the 1950's led to a further increase in blow molding demand, for applications such as liquid detergents, motor oil, water and milk. The lightweight HDPE one gallon milk container revolutionized the dairy industry, as glass bottles and paperboard were quickly replaced.

The production of polyethylene terephthalate (PET) led to the viability of reheat stretch blow molding. The strain hardening properties of PET allowed the high volume production of bottles able to resist the carbonation pressure in soft drink applications. The high clarity and economics of PET stretch blow molding have made this a popular production method for bottles for water, detergents, and other products.

Extrusion blow moulding (molding)
Parison extruded from an accumulator machine head. Courtesy Graham Engineering, York, PAIn Extrusion Blow Molding (EBM), plastic is melted and extruded into a hollow tube (a parison). This parison is then captured by closing it into a cooled metal mold. Air is then blown into the parison, inflating it into the shape of the hollow bottle, container or part. After the plastic has cooled sufficiently, the mold is opened and the part is ejected.

Plastic Packaging

Primary packaging
Bags-In-Boxes, Beverage Bottles, Plastic Bottles, Blister packs, Plastic Bubblewrap,Plastic bags
Plastic bottles, Skin pack, Plastic Wrappers

Secondary packaging
Shrink wrap

Tertiary packaging
Plastic Crates, Plastic Containers, edge protectors, Flexible intermediate bulk container, Big bag, "Bulk Bags", or "Super Sacks" , Intermediate bulk container, Plastic Pallets, Slip Sheet, Plastic Stretch wrap

Plastic Recycling
Plastic recycling is the process of recovering scrap or waste plastics and reprocessing the material into useful products. For instance, this could mean melting down polyester soft drink bottles then spinning the polymer into fibres.Before recycling, plastics are sorted according to their resin identification code. PET, for instance, has a resin code of 1.

Obstacles
When compared to glass or metallic materials, plastic poses some unique challenges from a recycling perspective. Chief among them is their low entropy of mixing, which is due to the high molecular weight of large polymer chains. Another way of stating this problem is that, since a macromolecule interacts with its environment along its entire length, its enthalpy of mixing is very, very large compared to that of a small organic molecule with a similar structure; thermal excitations are often not enough to drive such a huge molecule into solution on their own. Due to this uncommon influence of mixing enthalpy, polymers must often be of nearly identical composition in order to mix with one another. To take representative samples from beverage containers, the many aluminium-based alloys all melt into the same liquid phase, but the various copolymer blends of PET from different manufacturers do not dissolve into one another when heated. Instead, they tend to phase-separate, like oil and water. Phase boundaries weaken an item made from such a mixture considerably, meaning that most polymer blends are only useful in a few, very limited contexts.

Another barrier to recycling is the widespread use of dyes, fillers, and other additives in plastics. The polymer is generally too viscous to economically remove fillers, and would be damaged by many of the processes that could cheaply remove the added dyes. Additives are less widely used in beverage containers and plastic bags, allowing them to be recycled more frequently.

The use of biodegradable plastics is increasing. If some of these get mixed in the other plastics for recycling, the recycled plastic is less valuable.

Alternative processes
Many such problems can be solved by using a more elaborate monomer recycling process, in which a condensation polymer essentially undergoes the inverse of the polymerization reaction used to manufacture it. This yields the same mix of chemicals that formed the original polymer, which can be purified and used to synthesize new polymer chains of the same type. Du Pont opened a pilot plant of this type in Cape Fear to recycle PET by a process of methanolysis, but closed the plant due to economic pressures.

Another potential option is the conversion of assorted polymers into petroleum by a much less precise thermal depolymerization process. Such a process would be able to accept almost any polymer or mix of polymers, including thermoset materials such as vulcanized rubber tires and the biopolymers in feathers and other agricultural waste. Like natural petroleum, the chemicals produced can be made into fuels as well as polymers. A pilot plant of this type exists in Carthage, Missouri, using turkey waste as a feedstock. See the main article on thermal depolymerization. Gasification is a similar process, but is not technically recycling since polymers are not likely to become the result.

Recently, a process has also been developed in which many kinds of plastic can be used as a carbon source in the recycling of scrap steel.

Yet another process that is gaining ground with startup companies (especially in Australia, United States and Japan) is Heat Compression. The heat compression process takes all unsorted, cleaned plastic in all forms, from soft plastic bags to hard industrial waste, and mixes the load in tumblers (large rotating drums resembling giant clothes dryers). The process generates heat from the friction of the plastic materials rubbing against each other inside the drum, eventually melting all, or most of the material. The materials are then pumped out of the drum through heated pipes into casting moulds. The most obvious benefit to this method is the fact that all plastic is recyclable, not just matching forms. But criticism rises from the energy costs of rotating the drums, and heating the post-melt pipes.

Applications
The most-often recycled plastic, HDPE or number 2, is recycled into plastic lumber, tables, benches, truck cargo liners, trash receptacles, stationery (e.g rulers) and other durable plastic products and is usually in demand. The white plastic "peanuts" used as packing material are often accepted by shipping stores for reuse.

In Israel successful trials have shown that plastic films recovered from mixed municipal waste streams can be recycled into useful products.

Similarly, agricultural plastics such as mulch film, drip tape and silage bags are being diverted from the waste stream and successfully recycled into bulk resin commodities in Labelle, FL. Historically, these agricultural plastics have primarily been either landfilled or burned on-site in the fields of individual farms.

The environmental benefits of recycling plastic are that it produces less sulphur dioxide, less waste and less carbon dioxide.

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